Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a tower and a rotor arranged on the tower. The rotor, which typically comprises a hub and a plurality of blades, is set into rotation under the influence of the wind on the blades. Said rotation generates a torque that is normally transmitted through a rotor shaft to a generator, either directly or through the use of a gearbox. This way, the generator produces electricity which can be supplied to the electrical grid.
During operation of a wind turbine, its structure and components may undergo undesired movements, i.e. oscillatory or repeating displacements in any direction (fore-aft vibrations, side-to-side or lateral vibrations, longitudinal vibrations, torsional vibrations, . . . ) of different magnitudes and of different frequencies (high or low, constant or varying). These movements may be caused by different factors, e.g. by the wind pushing the wind turbine (i.e. wind thrust), blades locally disturbing the wind flow, vibrations transmitted from the gearbox to other components (e.g. tower), rotor movements, nacelle imbalances, vibrations from the hub transmitted to the tower etc.
If a wind turbine is subjected to the above mentioned collateral movements (e.g. vibrations) during a prolonged period of time, fatigue damage may result. Fatigue typically occurs with varying loads, e.g. alternating tension and compression. Fatigue may lead to a reduced life time of the wind turbine and/or its components. A further complicating factor is that the size of wind turbines (rotor, nacelle, tower, etc.) keeps increasing. Also, as wind turbines become higher, the effect of vibrations becomes more critical.
Furthermore, wind turbines are often grouped together in so-called wind farms. In a wind farm there may be a relatively short distance between wind turbines. Thus, action of the wind on one wind turbine may produce a wake which may be received by another wind turbine. A wake received by a wind turbine may cause high loads (particularly vibrations) and/or a reduction of electrical power production in this wind turbine. These high loads may damage components of the wind turbine, and these damages may reduce the life and/or the performance of the wind turbine.
One of the most impacting effects, which may potentially cause fatigue on the wind turbine, is the thrust exerted by the wind on the structure of the wind turbine. Wind thrust and its variation may depend on environmental (external) conditions and conditions inherent to the wind turbine itself (internal conditions). An external condition may e.g. be the wind speed, whereas internal conditions may e.g. be the pitch angle of the blades, the generator torque and the rotational speed of the rotor, etc. For example, a given wind speed faced with small pitch angles (of e.g. zero degrees) may cause a higher thrust than the same wind speed faced with greater pitch angles (of e.g. ninety degrees). Estimations of the thrust may be performed from parameters such as wind speed, pitch angles, rotor speed. Equivalently, a control of the thrust may be performed by suitably varying operational parameters of the wind turbine (e.g. pitch angles, rotor speed, generator torque).
WO2011157272A2 discloses a method of controlling a wind turbine having a rotor with pitchable wind turbine blades and a generator for producing power, wherein a pitch reference value for the wind turbine blades is determined, and an operational parameter representing a loading on the wind turbine rotor exerted by the wind is measured at time intervals. A variation parameter reflecting a variation of the operational parameter over time is determined and used in the determination of a minimum pitch limit value of the pitch reference value. The wind turbine is then controlled according to the pitch reference value only if the pitch reference value is above or equal to the minimum pitch limit value, and otherwise according to the minimum pitch limit value. The minimum pitch limit value may reflect the minimum pitch for maintaining the thrust on the rotor below or at a maximum allowable thrust level. One objective of maintaining the thrust on the rotor below or at this maximum allowable thrust level is to ensure that the loads on the turbine are kept within acceptable limits in all wind conditions.
The present invention aims at improving the prior art systems.